1. Field of the Invention
The present invention generally relates to an ultrasonic imaging method and an ultrasonic imaging apparatus, capable of performing nondestructive tests by employing ultrasonic waves. More specifically, the present invention is directed to such ultrasonic imaging method and apparatus, capable of scanning a radial-shaped area contained in an object to be inspected (sector scanning) by employing ultrasonic waves so as to obtain an image information of this radial-shaped area.
2. Description of a Related Art
Normally, in ultrasonic imaging apparatuses utilized as ultrasonic diagnostic apparatuses or industrial-purpose flaw detecting apparatuses, ultrasonic probes are employed each contains a plurality of ultrasonic transducers and has ultrasonic transmission/reception functions. In one typical ultrasonic imaging apparatus equipped with such an ultrasonic probe, image information related to an object to be inspected may be obtained in such a manner that this object to be inspected is ultrasonically scanned by using ultrasonic beams, while the ultrasonic beams are produced by synthesizing ultrasonic waves transmitted from the plurality of ultrasonic transducers. Then, the ultrasonic imaging apparatus may reproduce images of either two-dimensional areas or three-dimensional areas contained in the object to be inspected based upon the obtained image information. As one of scanning methods for scanning an object to be inspected by way of such ultrasonic beams, a so-called xe2x80x9csector scanning operationxe2x80x9d is carried out by which a two-dimensional fan-shaped region is ultrasonically scanned along angular directions.
FIGS. 7A to 7C are explanatory diagrams for illustratively explaining one typical example of the conventional sector scanning operation.
As shown in FIG. 7A, since ultrasonic waves transmitted to an object to be inspected from a plurality of ultrasonic transducers contained in an ultrasonic probe are synthesized with each other, an ultrasonic beam 101 is formed in the object to be inspected, while this ultrasonic beam 101 is extended from a transmission point 100 in a depth direction. Then, a fan-shaped two-dimensional area 103 which is contained in the object to be inspected is sequentially scanned by this ultrasonic beam 101 xe2x80x9cNxe2x80x9d times in a direction of an angle xe2x80x9cxcex8xe2x80x9d in an equi-interval. It should be noted that symbol xe2x80x9cNxe2x80x9d is a natural number.
Furthermore, as illustrated in FIG. 7B, at a plurality of sampling points 102, image information is sequentially sampled, while these plural sampling points 102 are distributed in an equi-interval in the depth direction along the ultrasonic beam 101 at the respective angles. As previously described, while the scanning operation by using one ultrasonic beam is carried out, image information related to a plurality of sampling points located on this single ultrasonic beam is sampled every time a predetermined time period has passed.
FIG. 7C shows a time chart for explaining such a scanning process operation of the ultrasonic beam. As shown in FIG. 7C, in order to perform a scanning operation of a single ultrasonic beam, a constant repetition time PRT (namely, pulse repetition time period) is consumed. Furthermore, a total value of pulse repetition time period PRT which is consumed to execute scanning operations of a plurality of ultrasonic beams-constitutes imaging time required for scanning an entire portion of a two-dimensional area. With respect to one pulse repetition time period PRT, a plurality of ultrasonic waves are transmitted to an object to be inspected so as to form one ultrasonic beam within a pulse transmission time slot TP. Then, at time instants indicated by white-colored points (see FIG. 7C), ultrasonic echoes are received which are reflected from a plurality of sampling points distributed along one ultrasonic beam, and then, image information related to the respective sampling points is sampled based upon these received ultrasonic echoes.
However, when such a sampling operation of the image information as shown in FIG. 7B is carried out, a total number of ultrasonic beams 101 (namely, density of ultrasonic beams) employed in a scanning operation of a unit area with respect to a deeper portion 105 within a two-dimensional area 103 becomes smaller than that of a shallower portion 104 thereof. As a result, an image quality of image information related to the deeper portion 105 becomes coarser than that related to the shallower portion 104.
As a consequence, as shown in FIG. 8A, the following solution method is conceivable. That is, since a total time of scanning operations for the deeper portion 105 is made larger than that of the shallower portion 104, density of sampling points 102 within the deeper portion 105 can be increased substantially equal to density of the sampling points 102 within the shallower portion 104. In FIG. 8A, black-colored points indicate such sampling points which are newly added. In this case, as shown in FIG. 8A, a scanning operation only directed to the deeper portion 105 is carried out between a first scanning operation and a second scanning operation of the conventional sector scanning operations, and then, such a scanning process operation is repeatedly carried out. FIG. 8B is a time chart for explaining such a scanning process operation of the ultrasonic beam. In this time chart, black-colored points represent time instants when ultrasonic echoes reflected from the newly added sampling points are received.
However, even in such a scanning operation directed only to the deeper portion, a time duration is required for ultrasonic waves transmitted from an ultrasonic probe to reach these sampling points and return to the ultrasonic probe. As a result, even when the scanning operation directed only to the deeper portion is carried out, such a time duration substantially equal to the time duration required for a single scanning operation in the conventional sector scanning operation would be consumed. As a consequence, a total scanning number of the sector scanning operation as shown in FIG. 8A is equal to substantially two times as large as a total scanning number of the sector scanning operation as shown in FIG. 7B. Thus, a frame rate of this sector scanning operation as shown in FIG. 8A, which corresponds to an inverse number of imaging time, would be lowered to a substantially half of a frame rate in the sector scanning operation as shown in FIG. 7B.
The present invention has been made to solve the above-described problems, and therefore, has an object to provide an ultrasonic imaging method and an ultrasonic imaging apparatus, capable of increasing density of sampling points in accordance with a depth degree within an object to be inspected, while a frame rate is not necessarily decreased.
To achieve the above-described object, an ultrasonic imaging method according to one aspect of the present invention, of obtaining image information in such a manner that a predetermined area contained in an object to be inspected is divided into at least a first area located most shallowly and a second area located deeper than the first area so as to scan the first and second areas by employing ultrasonic waves, comprises the steps of: (a) transmitting and receiving ultrasonic waves focused in one focus direction within the first area by using a plurality of ultrasonic transducers included in an ultrasonic probe to take samples of an ultrasonic image at a plurality of points in the focus direction, and changing the focus direction to scan the first area; (b) sequentially transmitting ultrasonic waves focused in respective focus directions within the second area by using the plurality of ultrasonic transducers in a predetermined time period, thereafter receiving ultrasonic waves reflected from the respective focus directions by using the plurality of ultrasonic transducers to take samples of the ultrasonic image at a plurality of points in the respective focus directions; (c) obtaining image information as to the plural points within the first area on the basis of detection signals obtained from the plurality of ultrasonic transducers at step (a); and (d) obtaining image information as to the plural points within the second area on the basis of detection signals obtained from the plurality of ultrasonic transducers at step (b).
Also, an ultrasonic imaging apparatus according to one aspect of the present invention, for obtaining image information in such a manner that a predetermined area contained in an object to be inspected is divided into at least a first area located most shallowly and a second area located deeper than the first area so as to scan the first and second areas by employing ultrasonic waves, comprises: drive signal generating means for delaying input signals to supply drive signals having specific phases, respectively; an ultrasonic probe having a plurality of ultrasonic transducers, for transmitting ultrasonic waves in accordance with the drive signals and for receiving ultrasonic waves to output detection signals based upon the received ultrasonic waves; signal processing means for processing the detection signals to obtain image information of an object to be inspected on the basis of the processed detection signals; and control means for controlling the drive signal generating means and the signal processing means to (a) transmit and receive ultrasonic waves focused in one focus direction within the first area by using the plurality of ultrasonic transducers to take samples of an ultrasonic image at a plurality of points in the focus direction, and change the focus direction to scan the first area and (b) sequentially transmit ultrasonic waves focused in respective focus directions within the second area by using the plurality of ultrasonic transducers in a predetermined time period, thereafter receive ultrasonic waves reflected from the respective focus directions by using the plurality of ultrasonic transducers to take samples of the ultrasonic image at a plurality of points in the respective focus directions.
According to the present invention, a radial-shaped area contained in the object to be inspected is divided into a plurality of areas having different depth degrees, and these plural areas are independently scanned. In particular, as to an area having a deep depth degree within the radial-shaped area, a plurality of ultrasonic waves are transmitted to the object to be inspected so as to sequentially form a plurality of ultrasonic beams extended in the different directions within a time period in which ultrasonic echoes reflected from an area located more shallowly than the deep area are obtained. As a consequence, density of ultrasonic beams in the deep area within the radial-shaped area can be increased, while the scanning time is not much increased. Accordingly, the density of the sampling points can be increased in accordance with the depth degrees of the object to be inspected, while the frame rate is not unnecessarily decreased.